When do we start talking about a nervous system ?

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This part of the lecture led us to discuss the very notion of the nervous system. This was done on the basis of light perception in unicellular organisms. We started from the idea of separating the light stimulus, which depends on photon-sensitive structures, from the analysis of this stimulus, which requires nerve centers.

We compared the structures of photon receptors in bacteria, unicellular eukaryotes and metazoans, thus introducing the question, still debated today, of the monophyletic or polyphyletic origin of the eye. We presented the opposing theories and, together with Walter Gehring, took a stand in favor of a monophyletic origin. This point was discussed again later in the lecture.

An example of a "primitive" eye is Chlamydomonas and its "eye spot", which functions as a visual apparatus with photoreceptor proteins that, in association with ion channels, regulate flagellar rotation and the movement of the unicellular towards the light or, on the contrary, in the opposite direction, depending on light intensity. Without going so far as to speak of a nervous system, we have here an organ that enables the unicellular to perceive a shadow, that of a predator for example, or to flee from a light intensity that is too strong, by activating ion channels and a motor system.

It is interesting to link these observations to Walter Gehring's hypothesis that light perception can be traced back to cyanobacteria, which integrated into eukaryotes in the form of chloroplasts. Gehring proposes that dinoflagellates, known symbionts of cnidarians, could have transferred their photoreception genes to cnidarians (in the same way as they incorporated those of cyanobacteria). Light sensitivity, born in cyanobacteria, would have passed to red algae in the form of primary chloroplasts, then to dinoflagellates and, from there, to cnidarians, whose position is very ancient in the phylogenetic tree.